Change the size of multiple selected polygons as a post-extrude operation
To create something like that initially, use individual inset (II):



Once the geometry is created, you can resize it with Edge slide (GG):



Note that the direction edges are slid is defined by what side the cursor is on when the operator is called, so if you want to make all the edges go in the same direction, make sure you have the cursor off to one side when you press GG.